An infrared remote control unit (generally simply called an infrared remote controller or a remote controller; hereinafter referred to as a remote controller) is a general equipment for a video device or an air conditioner to enable remote manipulation of the device. Normally, a remote controller modulates data of an infrared pulse sequence having a predetermined period, approximately 38 kHz, with an ON/OFF operation by a longer period, and then transmitting the modulated data to a remote control signal reception device such as a TV set.
Receiving the modulated data, the reception device demodulates the data, denoting ON with repeated pulses of about 38 kHz, into a signal with a smooth pulse from the beginning to end, before supplying the data to a decoding device.
In this way, the ON and OFF states are temporarily converted into digital signals of “H” level and “L” level, respectively, and then are supplied to a microprocessor provided inside the reception machine as remote control signals, and the remote control code of the signal is deciphered to interpret the command. PWM (Pulse Width Modulation) is a modulation mode using a rising point of the ON signal. PPM (Pulse Position Modulation) uses positions of a constant-sized pulse.
As may be seen, the period with a continuous pulse and the period with no discharge of pulse are very important in terms of proper decoding of the remote control signal in a later-stage circuit. Further, in PPM, pulse-size needs to be properly transmitted, otherwise accurate detection of the position fails.
Incidentally, conversion of the pulse into a smoothed signal denoting “H” level is generally performed by using a characteristic of the capacitor. Once change is accumulated in the capacitor, the output level dos not rapidly decrease even when the pulse turns OFF in a moment, as long as the capacitor holds the charge. Therefore, by appropriately setting the threshold, it is possible to digitally keep it in “H” level, as long as the pulse continues.
However, on the contrary, it is digitally maintained at “H” level even when the pulse ends until the charge of the capacitor is discharged, and the output level falls below the threshold. Accordingly, even when the transmission end transmits a regular ON-time signal, the “H” level signal in the receiving end becomes slightly longer due to the characteristic of the reception side. Further, as a result, the “L” level signal becomes shorter than its original regular signal. This defect is publicly-known, and is disclosed in, for example, Japanese Laid-Open Patent Application Tokukai 2001-145184 (published on May 25, 2001).
Here, the following describes conversion of the pulse into a smooth signal denoting “H” level, with reference to a part of FIG. 1.
In FIG. 1, the reference numeral 100 is an infrared pulse transmitted from a remote control transmission machine, that is generally a carrier of frequency of approximately 38 kHz (generally approximately 38 kHz-40 kHz). In the figure, the reference numeral 101 denotes a waveform having been smoothed by a smoothing circuit (not shown) including a condenser.
In the signal denoted by the reference numeral 101, the voltage gradually decreases in the portion corresponding to the trough of the infrared light pulse. However, the threshold value denoted by the reference numeral 103 (uniform voltage) is set so that the next pulse comes before the output voltage comes at or lower than the threshold value. After the pulse stops, the condenser carries out discharging at a certain time constant, and therefore it takes a little while for the voltage to decrease to “L” level (low-level).
The reference numeral 102 is a digitalized waveform of the output voltage (denoted by the reference numeral 101) of the smoothing circuit, created based on level comparison with the threshold value denoted by the reference numeral 103. As explained, it takes a certain time (denoted by the reference numeral 104) for the output voltage to decrease to or below the threshold value due to the characteristic of the condenser, and therefore, “H” level (high-level) period of the waveform 102 is extended by the period of the reference numeral 104, compared to the waveform denoted by the reference numeral 100.
To deal with such a characteristic, the receiving end needs to carry out decoding of the received signal in consideration of such difference with the original signal. In a general use, the problem may be solved in the remote control signal receiving end with a relatively simple arrangement corresponding to the degree of extension of the period of “H” level.
However, in a recent remote control technology, the signal transmitted from the remote controller is not simply processed directly in the general receiving end, but transferred to the receiving end via a separate device. For example, “Liquid Crystal FACE Wireless” (liquid crystal TV), a product of TOSHIBA, that became commercially available in September 2002, is equipped with a function called a ‘Remote Control Pass-Through (http://www.toshiba.co.jp/webcata/ctv/201f10.htm)”. In this function, the signal operated by a user using a remote controller is temporary received in a TV image reception machine, and then transmitted via a wireless medium to an image transmission machine or the like provided in front of a different device, such as a VCR.
This image transmission machine transmits the received remote control signal via the built-in remote control transmission machine while maintaining the original condition of the signal as much as possible. In the case of the foregoing example, it is assumed that the user watches a video image with a liquid crystal TV, that is distant from the VCR and the image transmission machine. When the user gives some kind of instruction, such as “pause” through the remote controller of the video, the instruction signal is temporarily received in the image/infrared signal reception machine included in the liquid crystal TV, and the signal is transferred to the image transmission machine by an electric wave. As a result, an infrared signal corresponding to the received signal is transmitted from a remote control transmission machine included in the image transmission machine. Then, the machine (VCR in this case) that received the infrared signal carries out operation of “pause”, as if the machine directly receives instruction of “pause” from the remote controller of the VCR.
The problem raised here is the error of the “H” level period in reception of the infrared light. A general structure assuming direct operation of the remote controller is designed with a margin to handle a stage of delay of relay transmission; however, the foregoing “Remote Control Pass-Through Function” requires the infrared light reception twice, thus increasing the error. The allowable range of the error differs for each case, and therefore, the user has to confirm proper operation of the system by referring to experiences, or by carrying out some kind of test. This problem is disclosed in the Japanese Laid-Open Patent Application Tokukai 2001-145184 in detail and some other publications.
To solve this problem, various methods have been proposed. For example, Japanese Laid-Open Patent Application Tokukaihei 04-302525/1992 (published on Oct. 26, 1992) discloses a pulse demodulation circuit that achieves reduction of the concerned false “H” period after termination of the pulse by using a constant-current load circuit and a limiter circuit. Further, the Tokukai 2001-145184 discloses a particular method to be used with the ‘Remote Control Pass-Through’ function.
However, the foregoing prior art has the following problems. The method of Tokukaihei 04-302525 achieves reduction of the concerned period; however, since the delay can still not be reduced to 0, this cannot completely solve the problem. For example, when the method is adopted for multi-stages relay operation, the error may be accumulated, causing some kind of defect.
Further, the method of Tokukai 2001-145184 gives up reduction of the concerned period, and instead intentionally causes a delay of the start time for the same length as the period, thereby matching the “H” level period with the time intended in the receiving end of the remote control signal.
However, to cause such a delay of the start time, this method requires adjustment of the capacitor, the resistor, and the reference voltage. Besides, such adjustment is performed in an analog manner, that may not be highly compatible with a digitalized device, and also may require a larger number of components.
Further, decipher of a remote control signal is generally performed by a CPU or a dedicated IC. Since, the capacitor or the resistor are not suitable to be included in these digital ICs, it increases manufacturing cost.
Furthermore, when such a circuit is provided in a semiconductor, the characteristic varies, and there will be some difficulties for adjustment. Since a recent technology uses various photoelectric conversion elements for converting an infrared light to an electric signal, those defects prevent the circuit from adjustment for each type of element.